Internal combustion engines emit a large amount of unburned hydrocarbons during cold engine start-up. In fact, a substantial fraction of the emitted hydrocarbons have been found to occur during the first ten of seconds due to the rich fuel mixture.
Release of hydrocarbons immediately after starting an engine poses a special problem because at this point the temperature of the conventional catalytic reactor is not high enough for conversion to innocuous products. The catalysts utilized in catalytic converter systems are generally ineffective at ambient temperature and must reach high temperatures, often in the range of 300.degree.-400.degree. C. before they are activated.
One method of reducing cold start emissions is to temporarily adsorb hydrocarbons on zeolites which desorb at engine operating temperature and are converted by the main body catalyst.
One disadvantage of this method is that specific zeolites do not adsorb with equal efficiency at the full temperature range of the exhaust gas at start-up, that is, from room temperature to about 300.degree. C. Additionally, the adsorbed hydrocarbons can prematurely desorb before the main conversion catalyst is up to operating temperature, thus emitting hydrocarbons to the atmosphere.
This problem is addressed in U.S. Pat. No. 4,985,210 which relates to an exhaust gas purifying apparatus which has a 3-way catalyst and either Y-type zeolite or a mordenite as an adsorbent for adsorbing a harmful component at the upstream side of the catalytic converter so that when the exhaust gas temperature exceeds the specific temperature, the harmful component desorbs from the adsorbent and is introduced into the catalytic converter. Further, an activated carbon trapper and a by-pass are provided in parallel at the upstream side of the adsorbent so that the flow paths of exhaust gas are selectively switched from one to the other in accordance with the level of exhaust gas temperature. Activated carbon is present for adsorbing at the lower temperature, the Y-type zeolite or mordenite is for adsorbing at the higher temperatures seen by the exhaust gas before engine operating temperatures normal for continuous operation are reached. The activated carbon is separate from the other adsorbing agents to prevent the activated carbon from being exposed to high temperature and eventually destroyed and to prevent excessive pressure drop in the exhaust system.
European patent application publication no. 0460542A2 relates to an engine exhaust system having two catalytic converters, a first noble metal catalytic converter positioned near the engine exhaust manifold and a second converter positioned farther from the exhaust manifold. The second converter contains molecular sieve (zeolite) materials capable of adsorbing and holding hydrocarbons to prevent their discharge into the atmosphere until the catalyst in the converters attains an efficient operating temperature. Because hydrocarbons begin to desorb from the zeolite at a temperature below light-off, it is not possible merely to place zeolite `in-line` in the exhaust system with the catalyst, the system employs one or more conveying means with several valves to selectively convey exhaust gases to each of the two catalytic converters in a manner such that initially produced hydrocarbon is withheld in the system by molecular sieves in order to be recycled through the converters and brought into contact with the catalyst after an effective converting temperature has been attained.
It would be desirable to have a way to efficiently adsorb hydrocarbons without premature desorption at engine start-up which would not result in destruction of the adsorbing agents.